1. Field of the Invention
The present invention relates generally to reactors involving gas-liquid-solid phases, and more particularly to a dispersed bubble reactor (DBR) that removes CO2 from gas streams through enhanced gas-liquid contact and mass transfer, which DBR is capable of handling relatively fine solid particles with relatively high throughputs.
2. Description of Related Art
Conventional gas-liquid contacting equipment in practice includes trickle-bed or packed towers, tray towers, venturi scrubbers, bubble column reactors and cyclonic contactors. Yet, these types of contacting equipment have limitations, including that none can achieve at least 90% CO2 capture efficiency handling precipitated solids.
For example, they cannot handle large volumes of gas such as in an integrated gasification combined cycle (IGCC) process, and in the flue gas CO2 removal process from a power plant. Shared characteristics of these processes are extremely large overall gas volumetric flow rates, and high CO2 concentrations in the gas-typically in the range of 10-50 percent (%) by vol %.
A case in point is the CO2 in the flue gas from a coal fired pulverized coal combustion (PCC) power plant, where the CO2 production rate is in the range of 1,600 to 2,200 pounds (lbs) for each megawatt-hour (MWh) of power generated. The best-known chemical solvent for this application is a monoethanol amine (MEA) and water solution mixture. The working (absorption) capacity of a 30 weight % MEA solution is in the range of 0.15 to 0.25 mol CO2/mol MEA. This relatively limited working capacity range dictates that for each pound of CO2 absorbed, 18 to 30 lbs of solution must be circulated between the absorber and the regenerator. Thus, for a 1,000 MW power plant, the required solution circulation rate is significant, and on the order of 30 to 66 million pounds per hour (lbs/hr), or 70,000 to 140,000 gallons per minute (gal/min). The flue gas production rate from such a plant is more than 2 million cubic feet per minute.
Such large gas and liquid flow rates create design challenges with conventional gas absorption technologies. Due to limitations in gas velocity, the packed bed or tray tower absorber requires an extremely large tower diameter (over 100 feet (ft) in inner diameter) if a single tower design is attempted.
Furthermore, the unprecedented size of a single tower, in and of itself, presents extraordinary challenges in uniformly distributing gas and liquid throughout the cross-section of a packed-bed absorber. The sheer size of the trays would make the tray tower implausible to design. If one were to implement a multiple tower design, the equipment required for distribution of the gas and liquid to the different towers is prohibitively expensive, as is the cost as a whole, escalating with each additional tower.
Particulates and solvents also present design hurdles. For most conventional solvents, particulates in the solution can lead to operational problems, such as foaming in the absorber. Severe operational consequences arise when the particulates deposit in the voids and cause blockages in the packing, as solvent flow becomes restricted. Conventional designs employ filters in an attempt to remove particulates in the circulating solvent that result from degrading solvent or corrosion product.
Solvents, operating conditions and working ranges carefully are chosen in conventional designs to minimize forming precipitating particulates due to absorption and reaction. Yet, conventional equipment severely limits the solvent choices and working ranges useful in the system, and specifically detrimental in improving CO2 capture process performance.
Large solvent circulation rates in the range of 30 to 66 million lbs/hr between the absorber and the regenerator with conventional processes necessitate significantly large amounts of steam energy to heat the solution in the regenerator. Over 25% of power generation is lost as a result both as energy is consumed by the solvent circulation pumps, and as steam is diverted from the steam turbine to regenerate the solvent.
In CO2 capture systems, it is highly desirable to have solids precipitate out from the solvent solution upon CO2 absorption and reaction in order to reduce the energy consumption with solvent circulation and rich solution regeneration process. One such analysis (Yeh, J. T. et al., Absorption and Regeneration Studies for CO2 Capture by Aqueous Ammonia, Third Annual Conference on Carbon Capture & Sequestration, May 3-6, 2004, Alexandria, Va., USA (“Yeh”)) shows that the lowest regeneration energy consumption can be achieved when the absorption and regeneration cycles can be represented with the following chemical reaction (1):(NH4)2CO3+CO2+H2O<=>2NH4HCO3  (1)
Ammonium carbonate solution, which has relatively high solubility in water, is used to absorb CO2 to form ammonium bicarbonate, which has a relatively low solubility and therefore precipitate out from the solution in the absorber.
If the absorber is packed with internals such as structured packing materials and distributors, the ammonium bicarbonate precipitates can deposit onto the packing and render the tower in-operable due to plugging. Yeh observed that ammonium bicarbonate can plug the pores of the sintered metal sparger in laboratory testing, even in spite of the CO2 flowing through the sparger. Hence, one can expect severe plugging problems with the packing material in the tower due to the low gas and liquid velocities, and to the existence of stagnant regions within the packing.
Similar problems due to plugging of passageways by solids in tray column absorbers are known. Another analysis (Brower, et al., Amino-acid Salts for CO2 Capture from Flue Gas, http://www.netl.doe.gov/publications/proceedings/05/carbon-seq/Poster%20147.pdf) shows that solids precipitation is highly desirable to increase the amino acid salt solution CO2 capture capacity, and to minimize overall energy needs of the CO2 capture process. For such solvent solutions that can cause solids precipitation, the conventional packed and tray towers will be unsuitable for CO2 absorption.
Venturi scrubbers and cyclonic spray type contactors can minimize or avoid internal packing, and potentially avoid problems associated with solids precipitation and deposition. A major characteristic of these types of contactors is the short contacting time, generally less than one second. However, for most known CO2 absorption solvents, the required gas-solvent contacting times are much longer. U.S. Pat. No. 7,862,788 to Gal et al. discloses that with 11 ft of packing, and even with piperazine as a promoter, the CO2 capture efficiency is relatively low in a range of approximately 15% to 72% and, without the promoter, in a range of approximately 8% to 35% within the ammonium carbonate and bicarbonate regeneration cyclone (NH3/CO2 molar ratio less than two). Gal et al. illustrates that the contacting time is insufficient. Therefore, venturi scrubbers and cyclonic spray type contactors cannot satisfy the long residence requirements to achieve at least 90% CO2 capture efficiency.
U.S. Pat. No. 5,342,781 to Su discloses an air-lift or bubble column reactor that can handle solids, and has a long gas-solids residence time. However, this type of the reactor has very low gas flow rates, and gas hold-up and the gas superficial velocity is generally less than 3 feet per second (ft/s). Low gas velocities means large reactor size, and at such low gas velocities, the reactor size must be sufficiently large to handle the large amounts of the flue gas from a power plant.
Further, as the CO2 absorption reaction is exothermic, it is necessary to remove the exothermic heat to maintain a low absorption temperature. With air-lift or bubble column reactors, low gas and solvent velocities make it difficult to remove the exothermic heat from the reaction zones. Even large air-lift or bubble column reactors, or multiple smaller reactors in combination, lead to a variety of problems, including poor gas and solvent flow distribution, channeling, inadequate heat dissipation and an inability to operate at low stable absorption temperatures.
A further challenge presented to successful systems that remove CO2 from flue gas is the allowable pressure drop or the power consumption in blowing the flue gas through the absorber due to the large volume. Such blowers consume large power even for small increases in pressure. Studies shows that the CO2 capture from a power plant can reduce the overall plant efficiency by approximately 9%, or in a relative scale by about 25% (see, http://www.fwc.com/publications/tech_papers/files/TP_CCS—10—04.pdf). Absorber designs have little margin for improvements in lowering the pressure drop.
In spite of the many challenges above, beneficial CO2 removal process incorporates contacting syngas from an IGCC process, or flue gas from a conventional PCC power plant, with a liquid solvent. Even with such gas-liquid contact systems, problems remain if cost and efficiency improvements are desired.
With some liquid solvents, high degrees of saturation with CO2 lead to formation of precipitates (crystals). Most gas-liquid contactors are incapable of handling precipitated solid particles. Other gas-liquid contactors do not have sufficiently long residence times to accommodate the relatively slower CO2 capture kinetics.
The inherently large size of conventional contactors for power plant presents design challenges in achieving uniformity throughout the cross-section. For some solvents, the solvent working (absorption) capacities need to be limited to avoid forming precipitates that can lead to operational difficulties with the conventional contactors. Such limitations on the working capacities of the solvent necessitate higher parasitic energy consumption during regeneration and solvent circulation between the absorber and the regenerator. As a result, conventional gas-liquid contactors, and the processes used therein, are too inefficient and too capital intensive to be useful in capturing CO2 from an IGCC process or from flue gas in a coal combustion plant for power generation.
What are needed are cost effective and reliable solutions for processing relatively large volumes of gases in relatively small reactors while also capable of handling precipitates formed in the process. It is to such systems and methods that the present invention is primarily directed.